Boron-containing ferrosilicon



BORGN-CQNTAINING FERROSILICON Application October 12, 1954, Serial No. 461,924

9 Claims. (Cl. 75-130) No Drawing.

This invention relates to a process for malleableizing iron, and to an alloy for effecting the same.

Malleable iron is a heat-treated product having greater strength and ductility than ordinary gray cast iron. This type of iron is cast in the form of white ironhaving carbcn present in the form of massive carbides of iron and manganese. Hard and brittle white iron is softened and toughened by an extended heat treatment at high temperature during which the iron carbide is dissolved and the carbon content precipitates in the form of temper carbon consisting of groupings of very fine graphite flakes. It is known that a small addition of boron to such iron makes possible its softening by annealing in a reduced period of time. This addition also makes possible the production of a more consistent and uniform malleable iron having improved characteristics of micro-structure, machinability and hardness. In addition, boron also increases the number of graphite nodules in the structure of the iron. Boron is also added to malleable iron for the purpose of neutralizing the carbide stabilizing effects of certain residual metals such as chromium, which may be introduced into the iron by steel or iron scrap in the furnace or cupola charge from which the iron is produced, the presence of which renders very difiicult the annealing of iron.

Hitherto it has been the conventional practice to add boron to malleable iron and other cast irons in the form of alloys of iron and boron. In this process the ferroboron alloy is added to molten metal as tapped into a ladle or alternately to metal in an air or electric furnace. The criticality of the amount of boron addition necessitates great care in operation. The quantities used being small fractions of 1 percent, very careful weighing is required if satisfactory control is to be obtained. Thus, if too little boron is added, a long annealing cycle will-be required to produce iron of the desired characteristics, while if too much boron is added, the ductility and other mechanical properties of the iron will be adversely affected. It is, furthermore, not practical to use ferroboron alloys in the cupola charge because of high losses occurring therein, which would render the boron treatment prohibitively expensive.

It is, therefore, the primary object of this invention to provide a ferrosilicon alloy containing boron, which alloy can be put through the cupola without subsequently treating the iron with costlier ferroboron alloy.

Another object of this invention is to produce malleable irons through the use of a boron-containing ferrosilicon alloy, in which the boron content is controlled within certain limits.

Another object is to provide improved control of annealing by the use ofboron-containing ferrosilicon, and to simplify the production of consistently high quality malleable irons of both the ferritic and pearlitic grades.

The means by which these objects are attained are based on the discovery that boron can be more easily and accurately introduced in cast iron to produce 2,77 8,732 Patented Jan. 22, rear ice malleable iron of highquality when it is alloyed with both iron and silicon. which element also promotes malleableizing, than when added with iron alone.

Ferrosilicon alloys containing between 15 percent and percent silicon with 0.01 percent to 0.20 percent boron are satisfactory for use in the production of malleable iron, and in the practice of this invention. The compositions of the alloys in regard to silicon and boron must be such that the'silicon to boron ratio would permit the addition of between 0.0005 percent and 0.003 percent boron without simultaneously adding too much silicon. In general, the boron content of the alloy must approach the lower end of the range specified in the alloys having the lower silicon contents. In cases where the silicon contents of the alloys are high, the boron content must be proportionately increased. Thus, by adjusting the composition of the boron-containing ferrosilicon, it is always possible to add sufficient silicon and boron to permt the addition of between 0.0005 percent and 0.003 percent boron to the iron.

The preferred range for the alloys is between about 25 percent and 90 percent silicon, with 0.01 percent to 0.20 percent boron, because with this higher silicon content an improved recovery of boron can be obtained. Satisfactory results, howeverjmay also be obtained with iron silicon alloys containing the lower percentages of silicon 'hereinabove mentioned.

As an example of the practice of this invention, pounds of 0.041 percent boron, 49.24 percent silicon ferroalloy was used per 10,000 pound cupola charge of duplex malleable iron, thereby introducing 0.00057 percent boron. A high recovery of boron was obtained, and the product annealed satisfactorily.

conventionally, the temperature used in malleableizing iron is in the range of l600 to 1750 F. As is Well known, the practice is to gradually heat the iron to this temperature, hold it there for the required time, and then slow y to cool it through the transformation range at a rate of less than 10 F. per hour to produce malleable iron free of the pearlitic constituent. In production, the annealing of malleable iron is carried on in cycles which vary extensively, depending upon the tonnage of metal being annealed, and upon the type of article so treated. The annealing cycle in some instances may be as short as 18 to 30 hours, while in others it may be protracted to 72 or more hours. By the careful control of the boron level, which is made possible'by the method of the invention, it is possible to cool the treated iron article at a faster rate.

No particular form of apparatus for preparing the molten mix or for pouring it, and no one particular type of annealing furnace are herein described by diagrams, inasmuch as any suitable prior art means for these purposes may be used.

The exact mechanism whereby the alloys of this invention operate to achieve the effects indicated is not known. The insoluble boron present in the annealed iron may be one of a number of compounds, but is believed to be in the form of either a nitride or carbide. it is thought that the insoluble boron compounds are rejected early during heat treatment of the iron, and act as nuclei for the growth of temper carbon during the annealing cycle. The presence of such a nuclei promotes the precipitation of temper carbon, thereby reducing the time necessary to maintain the process at the annealing temperature to obtain substantially complete precipitation of the carbon in the form of temper carbon. I

The ferrosilicon alloy containing boron can be used in lump form, or in the form of briquets. Briquets of the alloys produced from crushed alloy have been found to be especially suitable for malleable iron production. These can be prepared from a single ferrosilicon alloy containing boron, or may be prepared from ferrosilicon alloys or ferroboron alloys mixed prior to briquetting.

- In summary, the economy involved in using a ferrosilicon boron alloy of the type herein disclosed is of great importance, because boron can be introduced at a much lower cost. That this alloy may be added to the cupola charge eliminates the possibility of omitting by mistake ladle additions of the material. The addition of boron in the form of a high silicon alloy results in improved consistency of recovery, thereby yielding an iron which is more fool-proof from the standpoint of its response to the malleabilization heat treatment.

What is claimed is:

1. The process of producing malleable iron, which comprises adding to a cupola charge of iron, silicon and manganese, containing carbon and incidental impurities, an alloy consisting of silicon in the range between percent and 90 percent, between 0.01 percent and 0.20 percent boron, the remander iron; said boron thus introduced comprising between 0.0005 percent and 0.003 percent of said charge, forming a cast iron article and annealing said article at the annealing temperature to substantially convert and precipitate in the form of temper carbon the carbon present in said cupola iron.

2. The process of producing malleable iron, which comprises adding to a cupola charge of iron, silicon containing carbon and incidental impurities, an alloy consistng of silicon in the range between percent and 90 percent, between 0.01 percent to 0.20 percent boron, the remainder iron; said boron thus introduced comprising between 0.0005 percent and 0.003 percent of said charge, forming a cast iron article and annealing said article at the annealing temperature to substantially convert and precipitate in the form of temper carbon, the carbon present in said cupola iron.

3. The process of producing malleable iron, which comprises adding to a cupola charge of white iron an alloy consisting of 49.2 percent silicon and 0.04 percent boron, the remainder iron; forming a cast iron article and annealing said article at the annealing temperature to substantially convert and precipitate in the form of temper carbon the carbon contained in said cupola iron.

4. A process for producing malleable iron, which process comprises adding to a cupola charge of white iron briquets of an alloy consisting of silicon in the range between 15 percent and 90 percent, with between 0.01 percent and 0.20 percent of boron, the remainder iron, said briquets being prepared from mixtures of ferroboron and ferrosilicon alloys; forming a cast iron article and annealing said article at the annealing temperature to substantially convert and precipitate in the form of temper carbon the carbon contained in said White iron.

5. A process for producing malleable iron, which process comprises adding to a cupola charge of White iron briquets of an alloy consisting of silicon in the range between 25 percent and percent, with between 0.1 percent and 0.2 percent of boron, the remainder iron, said briquets being prepared from mixtures of ferroboron and ferrosiiicon alloys; forming a cast iron article and annealing said article at the annealing temperature to substantially convert and precipitate in the form of temper carbon, the carbon contained in said white iron.

6. An alloy which in its normal use is to be added to a cupola charge of iron, silicon, carbon and manganese in the production of malleable iron, which alloy consists of betweenlS percent and 90 percent silicon, 0.01 percent to 0.20 percent boron, the remainder iron.

7. An alloy which in its normal use is to be added to a cupola charge of iron, silicon, carbon and manganese in the production of malleable iron, which alloy consists of between 25 percent and 90 percent silicon, 0.01 percent to 0.20 percent boron, the remainder iron.

8. An alloy which in its normal use is to be added to a cupola charge of iron, silicon, carbon and manganese in the production of malleable iron, which alloy consists of 49.2 percent silicon, 0.04 percent boron, the remainder 9. An alloy which in its normal use is to be added to a cupola charge of iron, silicon, carbon and manganese in the production of malleable iron, which alloy consists of between 25 percent and 90 percent silicon, 0.1 percent and 0.20 percent boron, the remainder iron.

References Cited in the file of this patent UNITED STATES PATENTS 754,262 Weber Mar. 8, 1904 1,381,748 7 Rouse June 14, 1921 1,509,624 Walter Sept. 23, 1924 1,519,388 Walter Dec. 16, 1924 1,931,109 Ewing et al Oct. 17, 1933 2,280,286 Critchett Apr. 21, 1942 2,579,452 Eckman et al. Dec. 25, 1951 OTHER REFERENCES 

1. THE PROCESS OF PRODUCING MALLEABLE IRON, WHICH COMPRISES ADDING TO A CUPOLA CHARGE OF IRON, SILICON AND MANGANESE, CONTAINING CARBON AND INCIDENTAL IMPURITIES, AN ALLOY CONSISTING OF SILICON IN THE RANGE BETWEEN 15 PERCENT AND 90 PERCENT, BETWEEN 0.01 PERCENT AND 0.20 PERCENT BORON, THE REMAINDER IRON; SAID BORON THUS INTRODUCED COMPRISING BETWEEN 0.0005 PERCENT AND 0.003 PERCENT OF SAID CHARGE, FORMING A CAST IRON ARTICLE AN ANNEALING SAID ARTICLE AT THE ANNEALING TEMPERATURE TO SUBSTANTIALLY CONVERT AND PRECIPITATE IN THE FORM OF TEMPER CARBON THE CARBON PRESENT IN SAID CUPOLA IRON.
 6. AN ALLOY WHICH IN ITS NORMAL USE IS TO BE ADDED TO A CUPOLA CHARGE OF IRON, SILICON CARBON AND MANGANESE IN THE PRODUCTION OF MALLEABLE IRON, WHICH ALLOY CONSISTS OF BETWEEN 15 PERCENT AND 90 PERCENT SILICON, 0.01 PERCENT TO 0.20 PERCENT BORON, THE REMAINDER IRON. 